avcodec/rv34: Fix runtime error: signed integer overflow: 768 * 4126720 cannot be represented in type 'int'

Fixes: 1655/clusterfuzz-testcase-minimized-5587079276789760

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>